Tracer current detector



May 7, 1940. s. w. BORDEN TRACER CURRENT DETECTOR Original Filed April2, 1938 -mq mwM T Fig.5

Patented May 7, 1940 TRACER CURRENT DETECTOR Stephen W. Borden, Summit,N. J.

Original application April 2, 1938, Serial No.

Divided and this application September 22, 1939, Serial No. 296,049

4 Claims. (Cl. 175-483) This application is a division of my copendingapplication Serial No. 199,628 which matured into Patent No. 2,176,757on October 17, 1939.

This invention pertains to means for detecting the presence or absenceof a tracer current in an insulated and lead-encased conductor,regardless of the amount or the relative polarity of such tracer currentwhich may, at the same time, be flowing in the lead sheath surroundingthe conductor. The device is essentially a combination of the cablesheath current detecting device of my Patent No, 2,176,757 with aconductor current pickup device and additional equipment for combiningthe functions of the two mentioned devices.

In the drawing, which is schematic only, Fig.

. 1 represents the sheath current detector attached to a cable andarranged for independent operation, and Fig. 2 shows the sheath currentdetector arranged for cooperative operation with a conductor pickupcoil. Fig. 3 illustrates the wave form of the test current used in Fig.1.

Referring to the drawing, Fig. l. 24 is an iron core having means, asI2, for suspending it from a cable sheath. Wound on this core is a coil25 consisting of several turns which are insulated from each other. Forsimplicitys sake, the turns are here shown as air insulated but inactual construction they are insulated with ordinary insulating paper orcloth. The winding consists of a fiat braided copper conductor, the sizewhich I generally use being about wide by thick and having aconductivity substantially equal to that of No. 6 A. W. G. copper wire.This braided conductor extends beyond the ends of the coil, in onecontinuous piece, to whatever length may be desired, a length ofapproximately 4 ft. on each side of the coil being about the maximumdesirable. One end of the coil is attached to core 24, as by fastener 54which need not be insulated, while the other end of the coil is securedby fastener 53 to a strip of the insulating material 52 which, in turn,is secured to the core 24,

Also mounted on the core 24 is a high-resistance, high-voltage coil 28,the terminals of which are connected to posts 29 and 36 mounted on thestrip of insulating material 52. 2 is an insulated conductor containedwithin a lead sheath l and which is grounded to sheath I by a faultindicated by 3. Binding posts 8 and 9 areconnected to a source of swingcurrent. I4 is a transformer having a primary winding I5 and a secondarywinding I6; I I is a buzzer and I3 a one-way rectifier element. 46 is aswing meter, 45 a choke coil in series with the meter, 48 a volumecontrol, 44 a condenser, and I8 a telephone receiver.

The equipment is shown connected up for the purpose of detecting tracercurrent flowing in the sheath I and determining its relative polarity.For the purpose, I employ a swing current which, starting from post 8,flows to conductor 2 via rheostat 57, thence through the conductor tofault 3, thence through the lead sheath I to bond connections 4 and 6and thence through the earth to coil I5, and thence to post 9. I prefera tracer current of about 1.2 cycles per second and wave form similar tothat shown in Fig. 3. The braided conductor is attached to the leadsheath substantially as shown, the space between 26 and 21' being asgreat as may be feasible. The result of this connection is to shunt aportion of any sheath current which may be flowing in the sheath throughcoil 25, which produces a flux in core 24, which in turn produces apotential in coil 28, which in turn causes swing meter 26 I to swingback and forth in step with the swing current. Volume control 48 is usedfor keeping the current flow through the meter within proper limits. Sofar, we have equipment for detecting a flow of current in the cablesheathIbut not for determining its relative polarity.

Coil I6 of transformer I4 is in series with buzzer I I and rectifier I3,the whole being connected across posts 8 and 9. When the swing currentpolarity on post 8 is positive, current flows through rectifier I3,buzzer II and winding I3 which causes the buzzer to operate, which inturn produces an alternating current in coil I5 having a frequencydetermined by the speed of the buzzer, and the potential from coil I5superimposes the ripple 58 on the positive half of the swing currentcycle. When the swing current polarity reverses, the current will notflow through the rectifier I3 and consequently the buzzer does notoperate and there is no ripple superimposed in the negative half cycle.By connecting a telephone receiver I8 via condenser 44 across the coil28, this ripple is distinctly heard in the phone; and if it is heardwhen the needle on 46 is swinging toward the plus sign, it indicatesthat terminal 53 is positive for that half of the cycle, whichindicates, in turn, that the sheath current is flowing in the directionfrom 21 to 26 when post 8 is positive whereas if the ripple is heard onthe negative side of the meter swing it indicates that 53 is negativeand the flow is from 26 toward 27.

In locating a fault, as 3, a tracer current is introduced into conductor2 usually at theend ,where the conductor ordinarily receives itsoperating current, that is, at the supply end of the conductor, and thetracer current is considered to be flowing from the supply and towardthe fault, regardless of the fact that it may be an alternating current;and since tracer current which returns by way of the lead sheath of thecable always flows away from the fault 3, then if the points 26 and 21are on a part of the sheath which is between the end where the tracercurrent is applied and the fault, 53 will be positive whereas if 28 and21 are beyond the fault, as between 3 and 4, then 53 will be negative.

If, instead of swing current, plain direct current is used and amillivoltmeter be connected between points 26 and 21, the sameinformation may be obtained but there may be flowing in sheath directcurrent such as railway return current, whose magnitude may be manytimes that of the tracer current, with the result that the readings arerendered useless or there may be alternating current flowing in thesheath which might have the same effect. As fully set forth in my PatentNo. 2,176,755 issued on Oct. 17, 1939 the swing current test is notsubject to interference by either direct or alternating stray current inthe cable sheath; and the equipment as a whole is much more sensitivethan any other type of portable equipment having suflicent ruggednessfor the purpose.

The transforming device 24 is used for the double purpose of increasedsensitivity and for preventing a flow of direct current from the cablesheath into meter However, if the swing current in the cable sheath l,or in any other conductor, is of sufficient magnitude, its presence andpolarity may be detected by connecting meter 46 directly across aportion of the conductor, as shown in the dotted enclosure 58'. Whenconnected in this manner, a condenser M4 is inserted to block out directcurrent and the telephone I8 may be shunted across the choke coil 45.

The resistance of fault 3 varies greatly in different instances and maybe anything from a few ohms to several hundred thousand ohms; and,furthermore, it may vary greatly even during the test period; and this,in turn, means that the amount of tracer current flowing may vary overrather wide limits and also that it is convenient to make the tracercircuit voltage variable. These variables introduce considerabledifficulties in the way of superimposing a ripple current on one-half ofthe swing current wave, and for this and other reasons the equipmentshown in Fig. 2 is sometimes more desirable for the purpose of locatinga cable fault.

Referring now to Fig. 2. 5| is a portable meter device which includesthe swing meter 46 with its choke coil 45 in series. There are threebinding posts, 46, 4|, and 42. Post is connected to one side of meter 46via choke coil 45, and connected between 40 and 4| is a volume control47. Post 42 is connected to the other side of meter 46, and between 4|and 42 a second volume control 48 is connected. A single pole, doublethrow switch 50 is arranged to short-circuit either posts 40 and 4| or4| and 42. Posts 4| and 42 are connected via flexible leads 33 and 34 tothe terminals 29 and 30 of the coil 28 of a sheath current transformer24, and posts 40 and 4| are connected via flexible leads 3| and 32 toterminals 22 and 23 of coil 2| which is mounted on a flexible iron core20 which encircles the lead sheath i A detailed description of theconstructicn and operation of devices 20 and 2| will be found in myPatent No. 2,176,756 dated Oct. 17,

For locating the fault 3, tracer current from terminal 8 flows from theconductor 2 to the fault 3 from which it passes into lead sheath I.

We will assume that a 2-ampere tracer current is used and that oneampere flows through the lead.

sheath from 3 to the bond connection 6 from which it returns to post 9via ground I. The balance of the current flows from 3 to 4, thencethrough the ground 5 back to post 9.

The test is made as follows. Switch 50 is first thrown to position Cwhich short-circuits coil 28, leaving only coil 2| active. Rheostat 4!is adjusted so as to produce a swing on meter 46 of not more than halfscale. The switch is now thrown to position S which short-circuits coil2|, leaving coil 28 alone active, and rheostat 48 is adjusted to producesubstantially the same swing on meter 46 as that previously produced bycoil 2|. Switch 5|] is now opened, and if the polarity of coils 2| and28 are the same, the resulting swing on meter 46 will be greater thanthe swings on either of the coils individually, and if the polaritiesare opposite, the resulting swing will be approximately equal to thedifference. Another method of determining whether the coils are addingor bucking is to make the adjust: ments as previously outlined and thenthrow switch 5|] quickly from one position to the other and note whetherthe hand merely continues to swing or whether it comes to rest andreverses the direction of swing, which latter efiect will be present ifthe coils are bucking. This method will be found very effective when theconditions of test are such that it is possible to have only a verysmall deflection on meter 46. It will be understood that the magnitudeand polarity of the potential'of coil 2| is determined by the effectivecurrent threading coil 20 which current is the difference between thecurrent in conductor 2 and that flowing in the sheath.

The interpretation of the readings obtained from the meters is asfollows, having in mind that the purpose of the test is to follow thetracer current in conductor 2 from its point of origin 8 along the cableto the fault 3 for the purpose of locating the fault, it being obviousthat there will be no current flowing in conductor 2 at any point beyondthe fault. If neither coil produces a deflection on the meter, there isno current in conductor 2. If either coil produces a deflection and theother coil does not, there is current in conductor 2. If each coilproduces a deflection, then if the coil polarities are additive, thereis no current in the conductor; and if the polarities are opposed, thereis current in the conductor. All the foregoing applies whether the pointof test be ahead of the fault or beyond the fault, and it will be clearthat the current in the conductor may be traced regardless ofcounter-flow in the cable sheath.

While I have described certain features more or less in detail, it willbe understood that the invention will employ various devices ofdiffering at rheostat connected between 42 and 4|, a rheomounted upon aniron core which encircles the sheath, a transformer having aprimarywinding for shunting across a length of the sheath and asecondary winding, as 28, together with interconnecting leads forconnecting post 40 to a selected end of winding 2|, for connecting post42 to a selected end of winding 28, and for connecting the remainingends of the coils to post 4|.

2. Tracer current equipment for tracing current in a conductor enclosedin a lead sheath which includes a meter connected between two bindingposts, as 40 and 42; a third post, as 4|; a switchv for selectivelyshort-circuiting either posts 40 and 4| or 4| and 42; a conductor pickupcoil, as'2| mounted upon an iron core which en-- circles the sheath; atransformer having a primary winding for shunting across a length of thesheath and a secondary winding, as 28, together vwith interconnectingleads for connecting post,

terminal ofcoil 28 and a meter connected between the remaining terminalof coil 2| and the remaining terminal of coil 28; a single pole,

double'throw switch having its center point connected to theinterconnecting lead and one of its terminals connected to each of themeter terminals whereby the switchrmay be operated to selectivelyshort-circuit coil 2| or 28.

4. The method of determining the flow or absence of flow of a tracercurrent in a particular section of an insulated conductor encased in ametal sheath when a portion of the tracer cur-.

rent which is flowing in any section of the conductor is flowing also inthe metal sheath surrounding the particular section of conductor firstmentioned, which consists in deriving from the said particular sectionof conductor a potential responsive to the magnitude and polarity oftracer current flowing in said section and in the sheath surroundingsaid section and in deriving a potential from the metal sheathsurrounding said section, which potential is responsive to the magnitudeand polarity of any tracer current flowing in said sheath butnon-responsive to any flow in STEPHEN W. BORDEN.

